1. Field of the Invention
The present invention relates to an optometric apparatus appropriate for examining visual functions of an eye of an examinee.
2. Description of Related Art
There is an optometric apparatus for subjectively examining visual functions of an examinee's eye such as a refractive power by selectively placing optical elements having various optical characteristics to be disposed in a test window to present various kinds of optotypes through the test window to the examinee's eye. This type of optometric apparatus is constructed to change a prism degree (prism power) to be added to the eye, namely, to be disposed in the test window, thus enabling examinations of visual functions such as heterophoria, divergence, convergence, and so on. To provide a prism degree for the eye, the apparatus is provided with a pair of rotary prisms placed to be mutually independently rotatable about an examination optical axis of the test window.
In the above conventional apparatus, the rotary prisms are rotated by a pulse motor. However, a rotation step angle of a low-cost pulse motor is as large as 7.5°, so that a change step of the prism degree is as large as 0.1 prism. Further, a switch signal for changing the prism degree would be transmitted after a changed prism degree is displayed in an indicator or transmitted as a data signal representing for example the prism degree, thus taking much time to drive the pulse motor. In addition, the prism degree is changed intermittently in steps of 0.1 prism and therefore the prism degree could not be changed smoothly as compared with a manual optometric apparatus. Consequently, the above apparatus would be difficult to use in especially a divergence examination and a convergence examination which need smooth changing of the prism degree.